1. Field of the Invention
The present invention relates to image sensor arrays. More particularly, the present invention relates to CMOS image sensor arrays for still camera applications and to methods for operating those arrays.
2. The Prior Art
Integrated image sensors are known in the art. Such sensors have been fabricated from charge-coupled devices (CCDs) and as bipolar and MOS image sensors.
In the CCD imager art, on-chip frame storage capability has been employed previously. It has been motivated by the need to shift sensed charges out during a video frame time without letting them be contaminated by further exposure as the charges travel across the CCD array. Two storage techniques are commonly employed in the CCD imager art. According to the first technique, a second separate on-chip CCD array is provided under a light shield, and the entire image is quickly shifted along one dimension into the storage array, since shifting in one dimension is fast enough to avoid significant contamination. According to the second technique, line-storage CCDs are provided between the lines of sensors, with local light shielding. In the CCD art, techniques have been developed for preventing leakage and contamination due to minority carrier diffusion and leakage, but these techniques are not applicable to the CMOS sensor array because the silicon fabrication processes are different.
In still-camera applications with randomly-addressable CMOS active-pixel sensors, the problem of how to implement a short-exposure interval with a long readout interval exists. A typical active-pixel area-array image sensor is disclosed in Hurwitz et al., "An 800K-Pixel Color CMOS Sensor For Consumer Still Cameras", SPIE Vol. 3019, pp 115-124 and comprises a plurality of rows and columns of pixel sensors. The most common method of exposure for this type of sensor array is to cyclicly scroll through the rows so that the integration duration for each row is the same, but can be shorter than the total readout interval. This method of exposure control is known as an electronic shutter.
There are two problems with this type of electronic shutter. First, since each row scans a different time interval, there will be motion artifacts (the shape of moving objects will be distorted). In addition, this scheme requires a very high conversion rate analog-to-digital converter (ADC) implementation. For example, if the array has 1 million pixels, and the readout duration is 1/100 sec (about the maximum acceptable for a hand-held the required conversion rate is 100 million samples/sec. Since the state of the art for commercial ADCs with the required accuracy (10 bits) is about 20 million samples/sec, this means that a total of 5 ADCs would have to be used to allow for 1/100 sec exposures.
Another solution to the exposure problem is to provide a mechanical shutter for the camera. In this mode of operation, the entire array is first reset simultaneously. Then the shutter is opened for the duration of the exposure. After the shutter is closed, readout out can take place a relatively slow rate limited only by the dark current error in the pixels. However mechanical shutters add to the cost and complexity of the camera, and also contribute to camera shake.
Prior art in CMOS storage pixels has not yet addressed the problem of leakage and contamination, even though the problem has been noted. In the paper Yadid-Pecht et al., "A Random Access Photodiode Array for Intelligent Image Capture", IEEE Trans. Electron Devices vol. 38 no. 8 August 1991 pp 1772-1779, a prior-art storage pixel is described. The imager disclosed therein is motivated by the need to access pixel values in random order for certain processing functions, as opposed to being motivated by the need to have a readout interval longer than the exposure interval in high resolution still photography. Yadid-Pecht et al. describe the problems of "crosstalk" and "leakage" being "much stronger than predicted," but they do not offer any specific ideas on how to ameliorate these problems.
The pixel layout in this prior-art imager shows that the authors did not find a strategy for protecting the sensitive storage node from stray carrier diffusion, nor from light. They reference three papers from the CCD art for "technological solutions" to these problems, but the CCD art referenced does not obviously apply to the problem of CMOS storage pixels.
Another area of concern in electronic cameras is the ability to provide for the concept analogous to selectable film speed in conventional cameras (i.e., ASA or ISO film ratings). It would be advantageous if such functionality could be provided easily in a manner which does not adversely affect integrated circuit layout area for the imaging array.
It is therefore an object of the present invention to provide a pixel sensor and an array of pixel sensors that overcome some of the shortcomings of the prior art.
A further object of the present invention is to provide a storage-pixel sensor and an imaging array of storage-pixel sensors that overcomes the scanout problems inherent in prior-art imaging arrays.
Another object of the present invention is to provide an improved electronic shutter method for use with storage-pixel sensors.
Yet another object of the present invention is to provide a storage-pixel sensor and an imaging array of storage-pixel sensors that are compatible with the electronic shutter method of the present invention.
A further object of the invention is to provide a storage-pixel sensor and an imaging array of storage-pixel sensors that incorporate a "film speed" switching capability.
Another object of the invention is to provide a storage-pixel sensor and an imaging array of storage-pixel sensors that incorporate a "film speed" switching capability without adding significantly to the layout area of the imaging array.